The early antioxidative defence mechanisms were studied in Arabidopsis thaliana by applying a range of realistic Cd concentrations. Our data suggest that a 24 h exposure to 20 mu M CdSO4 is already too toxic to study moderate toxicity, whereas a highly coordinated oxidative stress-related defence response could be observed after Cd application of 5 and 10 mu M. Significant differences in transcript abundance of several genes involved in antioxidative defence were observed. The generation of superoxide seems the main cause of oxidative stress in the roots, whereas in the leaves hydrogen peroxide appears to be an important player. Furthermore, an increased transcript level of lipoxygenase, a potential inducer of oxidative stress, suggests a central role of this gene in causing the Cd-related redox imbalance. Our results show that Cd as a non-redox-active metal induces oxidative stress and indicate that the antioxidative defence system is moderated by the activation of different genes in different organs and cellular compartments.